Ambient Seismic Noise and Microseismicity Monitoring of a Prone-To-Fall Quartzite Tower (Ormea, NW Italy)

Chiara Colombero,Denis Jongmans,Alberto Godio

doi:10.3390/rs13091664

Abstract

Remote sensing techniques are leading methodologies for landslide characterization and monitoring. However, they may be limited in highly vegetated areas and do not allow for continuously tracking the evolution to failure in an early warning perspective. Alternative or complementary methods should be designed for potentially unstable sites in these environments. The results of a six-month passive seismic monitoring experiment on a prone-to-fall quartzite tower are here presented. Ambient seismic noise and microseismicity analyses were carried out on the continuously recorded seismic traces to characterize site stability and monitor its possible irreversible and reversible modifications driven by meteorological factors, in comparison with displacement measured on site. No irreversible modifications in the measured seismic parameters (i.e., natural resonance frequencies of the tower, seismic velocity changes, rupture-related microseismic signals) were detected in the monitored period, and no permanent displacement was observed at the tower top. Results highlighted, however, a strong temperature control on these parameters and unusual preferential vibration directions with respect to the literature case studies on nearly 2D rock columns, likely due the tower geometric constraints, as confirmed by 3D numerical modeling. A clear correlation with the tower displacement rate was found in the results, supporting the suitability of passive seismic monitoring systems for site characterization and early waning purposes.

Highlights

Continuous passive seismic monitoring has reached a decade of applications on gravitational movements of all types and geometries [1]
Examples of representative Probability Density Functions (PDFs) and time evolutions of Power Spectral Densities (PSDs) are shown in Figure 3 (S1) and Figure 4 (S2) in the 2–20 Hz frequency range
Ambient seismic noise recorded at S2 shows greater amplification with respect to S1 in two frequency bands located around 6 Hz (f1 in Figures 3 and 4)

Summary

Introduction

Continuous passive seismic monitoring has reached a decade of applications on gravitational movements of all types and geometries [1]. Passive seismic monitoring systems usually involve a set of spatially distributed sensors deployed on, around or inside the potentially unstable compartments. Spectral analysis and cross-correlation of the recorded noise are applied to extract resonance frequency variations and seismic velocity changes within the investigated volumes. Both seismic parameters can show reversible fluctuations driven by external modification in air temperature and precipitation [1] and irreversible drops in their values when failure is approached [2,3,4,5].

Methods

Results

Discussion

Conclusion